1. Field of the Invention
The present invention relates to a method of manufacturing a Z-directional compressibility gradient in a web of paper. More particularly, the present invention relates to a method which includes the application of a plasticizer to a web of paper for controlling the Z-directional compressibility gradient in the web.
2. Information Disclosure Statement
In the paper industry, calendering a web of paper is the process by which the surface of the dried web is smoothed in order to provide a surface that is suitable for subsequent printing, writing or coating.
Basically, the calendering step involves passing the dried web through at least one pair of counter-rotating rolls defining therebetween a calendering nip.
In view of the relatively high pressure exerted on the dried web passing through the calendering nip, the web is compressed and the fibers therein are compressed such that the resultant calendered web has a greater density than the web prior to calendering.
In an ideal situation, only the fibers in the vicinity of the surface of the web are compressed and made more dense while the fibers within the web towards the center of the thickness of the web remain relatively uncompressed. The aforementioned calendered web is desirable because it presents a smooth, dense printing surface while maintaining the necessary bulk and stiffness qualities.
In the prior art, water, and other cellulose plasticizers and various plasticizer solutions have been used to aid in the calendering process. Such processes have had limited success in developing a strong Z-directional moisture gradient in the sheet, and have also suffered from uniformity problems.
For example, in the bleached paper board industry, liquid is applied to uncoated paper board via the calender roll. A shallow trough of liquid is pressed against a calender roll to supply the moisture, the trough being known as a water box. The aforementioned method applies an excess of water to the paper board which often results in a flooded calendered nip in which the paper board absorbs as much liquid as it is capable of storing in its void structure. Such a system results in low paper roughness at the expense of stiffness loss and increased paper grammage to reach a given stiffness level
Another method that has been proposed is the use of steam boxes. Such steam boxes supply both heat and moisture to the sheet. However, because the steam is a low viscosity fluid, such steam deeply penetrates the paper structure and softens the inner fibers. The aforementioned calender steam treatment generally results in lower roughness, and correspondingly higher density at a given calendering condition.
Another prior art proposal is the use of sprayed moisturizers which offer many advantages but suffer from non-uniform water application due to the liquid droplet size and the difficulty in applying liquid into the depressions of the paper surface. Also, it is difficult to control the air flow near a spray system. Therefore, the object of the present invention is to provide a method of applying a uniform coating of liquid to a rough paper surface during or prior to the calendering process. The method involves the application of plasticizer such that the applied coating of plasticizer penetrates fibers near the paper surface such that these fibers can be easily compressed to impart smoothness to the paper while most of the interior of the paper sheet remains dry and relatively incompressible.
Another object of the present invention is the application of a plasticizer to a web which permits a uniform and controllable penetration of the plasticizer into the web such that the Z-directional compressibility of the web is controlled prior to the web extending through the calender nip.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings.